Formation mechanism and spectroscopy of C₆H radicals in extreme environments: A theoretical study

This study examined the reaction mechanisms of singlet (rhombic) and triplet (linear) C₄ with acetylene by using accurate ab initio CCSD(T)/CBS//B3LYP/6-311G(d,p) calculations followed by a kinetic analysis of various reaction pathways and computations of relative product yields in combustion and planetary atmospheres. These calculations were combined with the Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of reaction rate constants for predicting product-branching ratios, which depend on the collision energy under single-collision conditions. The results demonstrate that the initial reaction begins with the formation of an intermediate³i2 with an entrance barrier of 3.0 kcal mol^-1 and an intermediate¹i1 without entrance barriers. The product-branching ratios obtained by solving kinetic equations with individual rate constants calculated using the RRKM and variational transition-state theories for determining the collision energies between 5 kcal mol^-1 and 25 kcal mol^-1 demonstrate that l-C₆H + H is the dominant reaction product, whereas HC₃C₃ + H, l-C₆ + H₂, c-C₆H + H, and c-C₆ + H₂ are minor products. The electronic absorption spectra of solid neon matrices in the range of 17140-22200 cm^-1 were obtained by Maier et al., and the optimized ground and excited state structures of C₆H were used to simulate the absorption spectra by one-photon excitation equations. The displaced harmonic oscillator approximation and the Franck-Condon approximation were used to simulate the absorption spectrum of the B²Π ← X²Π transition of C₆H. This indicates that the vibronic structures were dominated by one of the six active completely symmetric modes, with v₃ being the most crucial.